The present invention relates generally to medical imaging devices, and more particularly, to a system and method using a parametric signal generator to reduce perceivable noise generated during operation of a medical imaging device.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but process about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization,” MZ, may be rotated, or “tipped,” into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated. This signal may be received and processed to form an image by application of a combination of linear gradient fields (Bx, By and Bz) as produced by the gradient coils. These fields cause the individual spins in the human tissue to precess at different frequencies (Larmor relationship) and these differences can be used to encode the raw data to provide real images.
As current is introduced to the gradient coils, such as to produce the Bx, By or Bz fields, an acoustic noise is created by Lorentz forces. This noise can be rather loud and might be described to those not skilled in the art as akin to beating of an empty drum with a hammer. While the production of noise in this manner does not directly affect the medical imaging process, the noise may be uncomfortable or disconcerting to an imaging subject. Accordingly, “noise cancellation” devices have been developed in an attempt to reduce the imaging subject's perception of the noise and thereby present a more comfortable environment for the subject during the imaging process. However, prior noise cancellation devices and methods have not met general acceptance for a number of reasons.
For example, attempts to utilize conventional sound production devices such as loud speakers to produce acoustic noise canceling signals designed to reduce an imaging subject's perception of noise have been largely unsuccessful for various reasons. First, conventional loud speakers become ineffective when subjected to strong magnetic fields such as those produced by the imaging process. That is, the magnetic field generated during the imaging process interacts with the voice coils in the loud speaker and interferes with proper emission of the desired noise canceling signal from the loud speaker. Second, conventional loud speakers emit audible signals that can be difficult to control as the audio signal disperses peripherally during propagation. As such, by removing the loud speakers from close proximity to the imaging device in an attempt to lessen the effects of the magnetic field produced by the imaging device, the audio may “bleedthrough” to undesired areas and may actually create more unwanted noise. Therefore, while extending the distance between the loudspeaker system and the imaging device lowers the effects of the field, the extended distance causes the noise reducing signal to further dissipate and disperse into unwanted areas.
Additionally, attempts have been made to construct noise reducing systems utilizing pneumatically driven or air driven signals, such as those found in commercial airline applications. These systems are advantageous because they can provide a highly directional signal to an area without the use of conductive materials that can be adversely affected by the magnetic field generated during imaging. However, pneumatically driven systems typically do not deliver signals accurately enough to sufficiently reduce noise generated by the imaging process. Therefore, while a headset may be made of plastic, glass, or some other non-conductive material such that signal delivery is highly directed and is unimpaired by magnetic fields, the accuracy of the signal delivered is insufficient to serve as a suitable noise canceling means.
Alternate audio producing systems such as piezoelectric speakers have also been found to be unsuitable for such noise canceling application due to inherent limitations at low frequency ranges. As such, though piezoelectric speakers are not impeded by the magnetic fields associated with medical imaging, suitable noise cancellation fails at the necessary low frequencies generated by the Lorentz forces on the gradient coils. Therefore, the low frequencies produced as a byproduct of the imaging process are unaffected and remain perceivable by the imaging subject.
It would therefore be desirable to have a system and method capable of generating suitable noise cancellation signals to reduce the perceived noise associated with medical imaging processes such as MR imaging. Furthermore, it would be advantageous to have a system and method capable of directionally controlling the emission of a noise cancellation signal to avoid unnecessary propagation of noise cancellation signals into undesired areas. Also, it would be desired that such a system and method be capable of generating the necessary noise reducing signal without substantial operational impairment by the magnetic field.